Method and apparatus for optimizing a steam boiler system

Abstract

A method for controlling a steam boiler or oil heater for maximum fuel efficiency by systematically finding the most fuel-efficient combination of input control values. A characteristic multi-dimensional look-up table is created by temporarily operating the process at all the possible functional combined settings of a plurality of input operators and recording for each combination of settings the resulting output values of a plurality of process parameters, for example, steam flow, steam pressure, and exhaust composition. Input combinations resulting in either non-functional process or unacceptable output values are eliminated. Steam flow rate is the primary output control parameter. A selected value of steam flow rate is the primary control setpoint for the process. If several combinations of input values can cause the process to meet the primary control setpoint, the combination using the minimum fuel flow is selected as optimal. If the desired setpoint does not correspond exactly to discrete input values in the table, the correct input settings may be inferred by interpolation. Valves and dampers are dynamically controlled by output drive signals in an improved closed-loop control, using a function of the process output value and time to recalculate and adjust the drive signals.

1. In a process having a plurality of input operators, each operator being independently variable over its own range of settings, and a plurality of process output parameters, each parameter having a range of acceptable values,
 a method for controlling the process such that a specified value for a primary process output parameter is achieved and all other process output parameter values are within their respective acceptability ranges, comprising the steps of:  
 a) characterizing said process to produce a look-up characteristic table by determining empirically the operational relationships between said plurality of input operators and said plurality of output parameter values for combinations of said input operator settings resulting both in operation of the process and in output parameter values within said ranges of acceptable values;  
 b) designating one of said process output parameters as a primary control parameter;  
 c) providing a desired value of said designated primary control parameter as a process control setpoint;  
 d) setting said input operators at a combination of respective settings as determined from said look-up table to cause said process to operate at a value of said designated control parameter approximating said process control setpoint.  
 
   
   
     2. A method in accordance with  claim 1  wherein said table includes a plurality of such combinations of input operator settings that can cause said process to operate at a value of said designated control parameter approximating said process control setpoint, and wherein an optimal one of said plurality of settings is selected based upon a process input criterion. 
   
   
     3. A method in accordance with  claim 2  wherein said process is selected from the group consisting of a steam boiler system and an oil heater system, said designated primary control parameter is selected from the group consisting of steam flow, steam pressure, and oil temperature, and said process input criterion is minimum fuel flow. 
   
   
     4. A method in accordance with  claim 1  comprising the further step of engaging close-loop feedback control means for said input operators to cause said process to operate at an output value of said designated control parameter matching said process control setpoint value. 
   
   
     5. A method in accordance with  claim 4  wherein said close-loop feedback control means includes a function of the process output and time to recalculate and adjust said drive signals to cause said process to come into control. 
   
   
     6. A method in accordance with  claim 4  wherein said process control means includes a computer. 
   
   
     7. A method in accordance with  claim 6  comprising the further step of calibrating said computer such that drive signals from said computer to said process operators produce a linear response in at least one of said operators. 
   
   
     8. A method in accordance with  claim 6  comprising the further step of adjusting said drive signals from said computer such that the instantaneous rate of change for each process operator relative to its total range of operability is the same for all such operators. 
   
   
     9. A method in accordance with  claim 8  comprising the further steps of:
 a) forming a table of process response time delays to said drive signals for each of said input operators as a function of system operating percentage;  
 b) when sending a drive signal to an input operator, determining from said table what said response time delay will be; and  
 c) waiting at least the length of said determined response time delay before sending another drive signal to said output operator, to minimize overshoot and oscillation of said process response.  
 
   
   
     10. A method in accordance with  claim 6  comprising the further step of causing said computer to check said process input and output parameters continuously against a thermodynamic model to determine when a process failure occurs. 
   
   
     11. A method in accordance with  claim 10  including the further step of using said computer to determine where in said process said failure has occurred. 
   
   
     12. A method in accordance with  claim 1 , comprising the further steps of:
 a) determining from said table values of said designated control parameter closest to and bracketing said desired value;  
 b) determining the interpolated position of said desired value between said bracketing table values;  
 c) using said interpolated position to interpolate between bracketing settings of corresponding of said operator input settings from said table; and  
 d) adjusting settings of said plurality of operators in accordance with said interpolations such that said process operates at said setpoint and values of all other of said output parameters are within their respective acceptability ranges.  
 
   
   
     13. A method in accordance with  claim 1  wherein said characterizing step includes the steps of:
 a) setting the positions of all input operators at predetermined limits of their operability ranges;  
 b) varying settings of a first of said operators in a plurality of discrete steps over its operability range while holding the settings of each of said other operators constant;  
 c) recording values of each of said output parameters at each of said discrete operator input settings;  
 d) changing the setting of a second of said input operators by a discrete step away from said operability limit;  
 e) repeating steps b) through d) in successive discrete steps until said second operator reaches the opposite limit of its predetermined operability range;  
 f) repeating steps d) and e) for each additional operator, whereby said characteristic multidimensional look-up table of operator input settings is created, as well as a database of parameter output values corresponding to each of said steps in said look-up table; and  
 g) deleting from said look-up table all input settings which fail either to cause the process to operate or to provide output values within said ranges of acceptable values, resulting in an adjusted look-up table of input settings under which the process will operate and will provide output values within said ranges of acceptable values.  
 
   
   
     14. A method in accordance with  claim 13  wherein each of said input operators is controlled by an electromechanical actuator responsive to drive signals from said feedback control means, and wherein each of said actuators is operable in discrete steps. 
   
   
     15. A method in accordance with  claim 14  wherein a discrete step encompasses an operability range from zero percent to one hundred percent. 
   
   
     16. A method in accordance with  claim 14  wherein each of said discrete steps encompasses between about one percent and about fifty percent of said operability range. 
   
   
     17. A method in accordance with  claim 1  wherein said input operators are selected from the group consisting of fuel flow valve, primary air flow damper, secondary air flow damper, trim air damper, feedwater control valve, main air blower, exhaust damper, flue gas recirculation damper, steam atomization valve, eductor fan for exhaust stack, boiler nozzle positioner, and combinations thereof. 
   
   
     18. A method in accordance with  claim 1  wherein said process output parameters are selected from the group consisting of steam flow, steam pressure, drum water level, primary blower speed, secondary air flow, trim air flow, combustion chamber pressure, exhaust carbon monoxide content, exhaust oxygen content, exhaust nitrogen oxides content, exhaust sulfur oxides content, exhaust gas flow, flue gas recirculation flow, input fuel stream BTU value, flame sensor, and exhaust temperature.